The relentless pursuit of smaller and smaller features in integrated circuits is approaching the limits of traditional photolithographic techniques. In these techniques, a photoresist is coated on a substrate, and a mask pattern is projected by electromagnetic radiation passing through the mask and onto the resist causing the resist to become exposed in a pattern corresponding to the features of the mask. The resist is then “developed” and rinsed in a solvent to remove a portion thereof and leave the photoresist, having the resulting projected mask pattern, on the surface of the substrate or on a film layer, such as a hard mask layer, located on the substrate. The material underlying the photoresist is etched, typically in anisotropic plasma based underlying film selective etching chemistry, to transfer the photoresist pattern into the underlying layer. Thereafter, the photoresist is removed by ashing or other removal techniques, and the substrate is wet cleaned to prepare it for the next process. However, feature sizes have shrunk below that which can be imaged (resolved) using these traditional lithographic techniques, and to extend the use of traditional lithographic techniques to form these features, subtractive techniques, such as double and triple patterning, have been employed. In these processes, to achieve the smaller feature sizes on the order of 40 or fewer nanometers, the hardmask may be patterned and used to etch an underlying patterning layer, and the hardmask removed and replaced with an additional hardmask, and the process of coating with resist, exposing through a mask, etching the hardmask, and then etching the patterning layer may repeated one or more times, to pattern the patterning layer before patterning the ultimate material layer in which the sub-40 nm features are formed. Nonetheless, despite these advances, current lithographic techniques will be insufficient to meet the reduced dimensions called for in future semiconductor technologies.
Modern photolithography depends upon the concept of a sacrificial “photo” exposed resist, wherein areas of the resist that are exposed to light behave differently than those which do not. Because these techniques rely on electromagnetic radiation, diffraction limits the smallest feature size that can be imaged or resolved. Additionally, even if the feature size may be imaged, the energy entering the photoresist may also be scattered therein, leading to irregular exposure of the resist across the depth of the resist. As a result, the feature size which is exposed will actually be larger than the smallest resolvable image, and, it will have non-uniform sidewalls or other irregularities.
Currently, high-volume manufacturers are using deep-ultraviolet (DUV) photons, such as photons with a wavelength of 193 nm, to expose a photoresist material. Manufacturers are also using liquid immersion techniques and techniques such as the aforedescribed multiple patterning, to create patterned features on substrates of a small size.
Recently, laser beam lithography has again been investigated as a mechanism to expose very small features in photoresist and thereby break through the resolution limitations of traditional mask based photolithographic techniques by creating small cross section, high power laser beams. Fischer and Wegoner, in Laser and Photonics Reviews, 7, No. 1, 22-44 (2013) discuss an idea of using a stimulated emission depleted (STED) beam to expose the resist three dimensionally, i.e., in a columnar fashion, and thus direct laser write a feature through the full depth of the resist. In order to reduce the effective cross section of the resist which is exposed to sufficient energy to be polymerized thereby, two laser beams, a “normal” beam and a depletion beam, are used to form a STED beam. The excitation beam excites the polymer in the photoresist to cause it to polymerize, and the depletion beam reduces the energy in the photoresist before the photoresist polymerizes, thus keeping the photoresist from polymerizing where the depletion beam and the excitation beam energies overlap. Improved resolution occurs where the spatial maximum of the excitation profile of the normal beam corresponds with the local zero of the depletion profile of the depletion beams, i.e., the depletion beam is configured to spatially surround the excitation beam, and cull, from the resulting exposed regions of the photoresist, the polymerizing effect of the skirt region of the Gaussian beam. As a result, the portion of the excitation beam in which the polymerizing reaction is not cancelled by the depletion beam has a very sharp, nearly rectangular energy profile across its width rapidly reducing at the edges of the profile to an energy level below the polymerization energy of the resist, such that a sharply defined higher energy region is formed in the beam as opposed to traditional, Gaussian profile, beams. However, even using this direct laser beam exposure system, repeatable and sharply defined features smaller than 20 to 30 nm are difficult to achieve, in part because of the limit in the size of the sharply defined region of the beam, and in part because of inherent migration of polymerization in the photoresist being exposed from the location where the beam enters the photoresist into adjacent locations.